What Factors Affect the Rates of Chemical Weathering?

Composition of Rock

• The type of rock is the primary factor affecting the rate of chemical weathering. Rocks that are porous tend to weather much more quickly, because the amount of surface area exposed to the elements is increased. Because quartz is not very porous, it tends to weather slowly. For the same reason, rocks that are composed of large grains weather more quickly than rocks composed of small grains. Lastly, the presence of iron in rocks will cause those rocks to weather relatively quickly, because iron oxidizes (commonly referred to as "rusting").

Physical Weathering

• Anything that physically weathers rocks will increase the speed with which chemical weathering occurs. This is because physical weathering, by cracking and breaking apart rocks, increases the surface area that is exposed to the processes by which chemical weathering occurs. One of the major culprits is vegetation. Tree roots commonly break apart rocks, as do smaller plants such as grass and lichens. Areas with heavy vegetation tend to undergo chemical weathering at faster rates.

Climate

• The climate has a pronounced effect on the rate at which chemical weathering occurs. Because the presence of water is necessary for chemical weathering to occur, more humid areas tend to be the stage for faster rates of chemical weathering. Temperature is another major factor that speeds the rate of chemical reactions. According to Professor Michael Pidwirny of the University of British Columbia, "tropical weathering rates are three and a half times higher than rates in temperate environments."

Acid Rain

• "Acid rain" is rainwater that has a more acidic pH than normal. Acidic rainwater is caused by the release of chemical pollutants by human activities. In some areas, acid rain contributes to a relative decrease in the rates of chemical weathering, because it kills vegetation that would otherwise speed rates of weathering. On the other hand, acidic rain greatly increases the effects of chemical weathering on rocks such as limestone, and generally increases the rate at which the chemical weathering processes of carbonation and hydrolysis occur.